This invention relates in general to automatic gauge control for a rolling mill.
It is known in a rolling mill to measure the gauge of a strip of material being rolled after it has passed through the work rolls of a stand and to use an automatic gauge control system responsive to a gauge error so detected in order to modify the speed at which the strip is being fed into the stand, thereby to correct for the gauge error.
It is also known in a multi-stand rolling mill to use two automatic gauge control systems, one at the delivery end with a gauge error being measured after the last stand, the other at the entry to regulate the strip gauge when leaving the first or the second stand.
The present invention relates more particularly to automatic gauge control at the entry of a rolling mill.
Automatic gauge control is usually conducted with a predetermined, or normal, strip tension between the stands. When the interstand tension becomes too high, or too low, due to variations in the mill set-up, or in the incoming product, the automatic gauge control system becomes unable to regulate the gauge and additional means are necessary to control the gauge. U.S. Pat. No. 3,782,151 of R. S. Peterson et al, issued Feb. 29, 1972 discloses range control means associated with an automatic gauge control system and activated, when there is an out-of-range tension condition, to bring the tension between the last two stands within the limits of control.
In the aforementioned Peterson patent, for normal interstand tension the delivery gauge is regulated by adjusting stand speed at the last stand through a tension regulator, and an out-of-range condition is corrected by adjusting the speeds of several stands directly through a range control device.
Range control correction at the entry of the rolling mill involves tension between the two first stands and the corrective mode is necessarily different from what is needed at the delivery end. An interstand corrective loop may be provided between the first stands as suggested in U.S. Pat. No. 3,507,134 of A. V. Silva. The tension is then automatically adjusted to the desired value by sensing intertension error with a tensiometer and applying a corrective signal to the screwdown system of the next, or the preceding stand. However, such loop does not provide any direct response to gauge variation.
Strip gauge error is generally corrected by screwdown correction to a stand preceding the location of the gauge sensing device. This technique is disclosed in U.S. Pat. No. 3,492,844 of A. V. Silva. The U.S. Pat. No. 3,492,844 of Silva shows how gauge correction by screwdown speed control is sensed after a time delay corresponding to transport time from the roll bite of the stand to the location of the X-ray gauge. Also the patent shows an automatic adjustment of the corrective gain to the size of the gauge error.
As required by the constant mass flow of metal through the stands of a multi-stand rolling mill, roll gaps and the speeds of the stand are maintained in a proper relation between each other throughout the mill. Therefore, an automatic gauge control (AGC) system on the mill operates only for a small fraction of the stand top speed, typically .+-. 10%. Whenever an excessive correction is demanded by the system, the AGC saturates and no further correction can be made. For the same reason an additional correction by stand speed is not desirable at the entry of the rolling mill since it would affect the operation of all the succeeding stands.